Specialized furnaces are known in the art for use in pyrolysis systems, in a number of different operations. These include, for example, thermal and catalytic cracking of hydrocarbons, reforming operations, etc. Hereafter, the term cracking furnace will be used, although the principles disclosed are not limited to cracking operations. Such furnaces typically have a firebox whose walls define a combustion volume, and through which pipes extend, configured to carry a working fluid or product fluid through the combustion volume. The fluid in the pipes is the primary heat load that receives thermal energy produced within the firebox. Cracking furnaces generally employ multiple burners within a combustion volume in order to produce a uniform vertical heat flux profile, for efficient transfer of heat to the load. Many furnace designs employ arrays of wall burners distributed on opposing walls, or on all of the vertical walls of the firebox, with the heat load positioned between the arrays of burners. Cracking furnaces may also employ similar burners on top and bottom walls.
FIG. 1 is a side-sectional diagram of a portion of a cracking furnace 100, according to known principles. The furnace 100 includes a combustion volume 102 defined in part by a wall 104 with a lining 106 of refractory material. The refractory lining 106 can be in the form of bricks, tiles, large panels or slabs, etc., and typically lines all or nearly all of the walls 104 that define the combustion volume 102. Wall burners 108 are positioned in apertures 110 in the wall 104 and extend into the combustion volume 102. In the example shown, Venturi fuel jets 112 draw air into nozzle bodies 113 of the burners 108 and the fuel-air mixture is emitted from nozzle slots 114 that are spaced radially around an end portion of the wall burners 108. Flames 116, supported by the fuel-air mixture emitted from the nozzle slots 114, extend outward from the wall burners 108 in an X-Z plane lying substantially parallel with the wall 104. The flames 116 heat at least the exposed inner face 120 of the refractory lining 106 to an incandescent temperature, causing the wall 104 to produce thermal radiation 118 which propagates toward the interior of the furnace 100, where a heat load is positioned.
The majority of the thermal energy received by the load is transmitted by thermal radiation 118 emitted by the refractory lining 106, with a much smaller portion transmitted by convection currents of gaseous fluids inside the combustion volume 102, which are heated by contact with the inner face 120 and with the flames 116.
The heat load is generally positioned in the center of the combustion volume 102, with wall burners 108 positioned on two opposing walls, or on all four walls. In some systems, burners of similar design are also positioned on the floor and/or ceiling of a furnace. Some cracking furnaces are cylindrical, with the heat load positioned along the central axis, and wall burners surrounding the load on the cylindrical walls.
FIG. 2 is a side view of a portion of a cracking furnace 200 of another design, according to known principles. The furnace 200 includes a combustion volume 102 defined in part by a wall 202 that includes tiles or panels 204 of refractory material, fixed to the wall 202 at an angle, relative to a vertical axis Z. Wall burners 206 extend into the combustion volume 102 via apertures 110, and include vertically-oriented nozzles 208. In the example shown, the apertures 110 include angled portions 210 that emerge from a top face 212 of the refractory panels 204. In other cases, the panels 204 may be notched, a space may be provided between the panels, or other structures may be used to enable placement of the nozzle 208 in a position that generally corresponds to that shown. The wall burners 206 of the pictured example each include a fuel conduit 214 and an air conduit 216, through which fuel and air are introduced into the furnace 200. The fuel is emitted from the nozzle 208 while the air is introduced adjacent to the nozzle 208, where it can be entrained by the fuel exiting the nozzle 208. The mixture of fuel and air supports flames 116 that burn along faces 218 of the tilted panels 204. The inner faces 218 of the panels 204 are heated by the flames 116 to incandescence, emitting thermal radiation 118 toward a heat load within the combustion volume 102.
The wall burners shown in FIGS. 1 and 2 are only two examples of a number of wall burners that are known in the art, and employed in various types of furnaces.